This invention relates to lenses for eyeglasses, and particularly to bi or multi focal lenses.
Known lenses of this type include a surface having an upper distance-vision portion or distance portion FT, a lower near-vision portion or near portion NT and a varying or progressional or transitional region PB between the two portions. The near portion is sometimes called the "add".
In the upper distance portion FT the surface contains a far reference point B.sub.F and exhibits predetermined average distance-vision surface refractive powers D.sub.F. The lower near portion NT, containing a near reference point B.sub.N in accordance with DIN 58208, has predetermined average near-vision surface refractive powers D.sub.N. The transitional region PB has average surface refractive powers D.sub.P which effect a smooth transition from the distance portion FT to the near portion NT. The surface is divided into a temporal portion and a nasal portion by a principal meridian M which forms an umbilical point line.
An object of the invention is to provide a surface for such a lens for eyeglasses which has the following properties:
(a) a large, almost spherical, distance-vision portion in which the effect varies, at most, .+-.0.1 diopters,
(b) a large, also almost spherical, near-vision portion, in which the effect varies, at most, .+-.0.1 diopters,
(c) a progression region whose peripheral surface astigmatism is small and whose average effect in the horizontal direction exhibits only a small gradient and whose length along the principal meridian is approximately 18 mm.
The terms used in this application, the prior art, the objects of the invention, features of the invention, the advantages of these features, and details of preferred embodiments can be best understood from the following description when read in light of the drawings: